Spray painting of irregularly shaped articles by atuomatic production means presents a substantial problem in achieving an adequately uniform coating of the workpiece surfaces. When utilizing non-electrostatic spray coating techniques, the automatic spray coating of a part having a variety of surface exposures may require a complex initial setup of the spray heads, to direct sprays from various angles and in various directions, in order to be assured of applying coating material to all of the surface exposures. In this respect, it will be understood that, with non-electrostatic spray techniques, surfaces must be "seen" by the spray heads, in order to be coated.
Where a long production run of identical parts is to be spray coated, extensive and complex setups may be justified, in order to effectively utilize non-electrostatic spray coating techniques. However, where workpieces of a variety of sizes and shapes must be accommodated in the spray coating system, it may not be practical or economically justified to utilize complex nozzle arrangements, requiring significant setup time. In such cases, it may be necessary to spray the primary surfaces with an automatic system and, in addition, to resort to extensive manual touch up in order to complete the job.
For some applications, electrostatic spray coating techniques may be utilized in order to take advantage of the "wrap around" effect of the electrostatically attracted paint particles. However, electrostatic coating techniques themselves have important limitations, particularly in connection with extremely irregular workpieces. The electrostatic wrap around effect is relatively limited in nature, and does not obviate the need for special setup of the spray heads for reaching deep recesses and other difficult-to-reach surfaces.
The present invention provides a unique method and apparatus for achieving a highly satisfactory spray coating of irregular workpieces which, within practical limits, is able to process workpieces of extremely large size, having surface areas exposed along the various axis. Pursuant to the invention, one or more spray heads is mounted for rotary movement about a predetermined axis. The spray head or heads are in turn disposed at a substantial angle to the rotational axis (typically around 45.degree.) such that, when the spray heads are rotated about a horizontal axis (for example) the spray fans emanating from the spray heads will, at one time or another, traverse most surfaces which are either in front of, above or below, or to either side of the rotating spray heads. In an automatic spray coating line, the workpiece to be coated is conveyed through a spray coating chamber past a coating station at which the rotating spray heads are located. Pursuant to the invention, the spray heads are reciprocated more or less at right angles to the path or axis along which the workpiece is conveyed, so that the rotating nozzles sweep the work from end to end and also from top to bottom.
According to one aspect of the invention, predetermined relationships are maintained between the average radius of the spray fan intercept relative to the axis of rotation, the rate of rotation of the nozzles, and the relative translation thereof which occurs as a result of the combined effects of horizontal conveying of the workpiece and vertical reciprocation of the rotating spray nozzles, such that the path described by the spray fans, as applied to a flat surface normal to the rotating axis, is a tightly closed prolate cycloid. Inasmuch as the path described by the rotating nozzles is relatively complex, it is significant to the process that there be an appropriate correlation of angle of spray nozzle orientation, spacing of spray nozzles from the work, rate of rotation of the nozzles and rate of relative translation thereof, in order to achieve the desired path of spray application. Where the proper relationships are not observed, spray coating application on the workpiece surfaces may be highly irregular and of unsuitable quality.
Although rotary motion of the angularly disposed spray heads is a significant aspect of the invention, the rotary motion need not be continuous and unidirectional, but may be of a oscillatory nature. Utilizing a pair of nozzles rotating about a common axis, for example, it may be advantageous to rotate the assembly for 180.degree. in one direction and then 180.degree. back in the opposite direction. This has certain practical advantages in eliminating the need for rotating seals, which can be difficult to maintain in a painting system. It also has the effect of reversing the orientation of the cycloidal curve with each reversal of direction of rotation of the nozzles. Regardless, however, of whether the nozzle rotation is undirectional or reciprocating, the various abovementioned geometric relationships are so adjusted and related that the ratio of the mean diameter of the spray application path to the diameter of the "equivalent" rolling circle is at least about five to one, in order to generate the desired, tightly closed prolate cycloid coating path.
Although the invention may not be exclusively limited to such utilization, it is extraordinarly advantageous when used in conjuction with a recirculating overspray recovery system of the general type described in the E. O. Norris U.S. Pat. No. 2,848,353. With this system, the spray material may be applied substantially without regard to the amount of overspray (spray material not contacting the workpiece) inasmuch as such spray material is collected and recycled. The combination of the rotating, reciprocating spray heads, as above described, in conjunction with the recirculating and overspray collection system, is outstandingly advantageous for the spray coating of workpieces in which there is a considerable amount of open work, for example, or where the general configuration of the part is highly irregular from place to place such that a reciprocating stroke adequate for some portions of the workpiece would tend to be excessive for others. Particularly in the spray coating of open truss work, for example, the spray nozzles are necessarily spraying into open air a substantial percentage of the time. In such cases, in the absence of a recirculating overspray collection system, the economics of automatic spray coating might be prohibitive, notwithstanding the highly superior quality of the coating results achieved.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description and to the accompanying drawings.